Because of the different powers in the distance and near portions, different magnifications necessarily result in the near and distance portions, which are disturbing to the spectacles wearer.
In EP-A-0 809 126 a progressive spectacle lens is described in which the differences of magnification between the near and distance portions are to be reduced.
For this, formulae are used for the computation of the magnification which—as has been realized in the present invention—are not suitable for computing the magnification at any desired position of a progressive spectacle lens. Namely, the basis of the formulae used in EP-A-0 809 126 is a centered optical system in which the principal ray passes through the spectacle lens without being refracted. For this, attention is drawn to the textbook by Josef Reiner “Auge und Brille” (=“Eye and Spectacles”), to which reference is expressly made concerning an explanation of all terms which are not described here in more detail.
Before dealing more closely with the invention, attention will be given to the formulae used in EP-A-0 809 126:
The classical formulae as used in EP-A-0 809 126 for the magnification ┌ of a centered afocal system, are:       Magnification    ⁢                   ⁢    Γ    =      N    =                            tan          ⁢                                           ⁢                      w            ′                                    tan          ⁢                                           ⁢          w                    =                                    h                          f              2              ′                                            h                          f              1              ′                                      =                                            f              1              ′                                      f              2              ′                                =                                                    f                1                ′                                                              f                  1                  ′                                -                e                                      =                          1                              1                -                                  e                  ⁢                                                                           ⁢                  D1                                                                              Scale    ⁢                   ⁢          (      lateral      )        ⁢                   ⁢          β      ′        =                    y        ′            y        =                                        f                          2              ⁢                                                                     ′                    ⁢                                           ⁢          tan          ⁢                                           ⁢          w          ⁢                                           ⁢          o                                      f            1            ′                    ⁢                                           ⁢          tan          ⁢                                           ⁢          w          ⁢                                           ⁢          o                    =                                    f            2            ′                                f            1            ′                          =                  1          Γ                    
Total magnification Ng of an afocal System consisting of a fully corrected spectacle lens and a refraction deficit       Ng    =                  1                  1          -                                    e              _                        ⁢                          D              Br                                          =              1        +                              e            _                    ⁢                                           ⁢                      A            R                                    Ng    =          Ne      ·      Ns      wherein:    DBr is the power of the spectacle lens    AR is the distance point refraction of the eye    ē is the distance between the image side principal plane of the spectacle lens and the object side principal plane of the eye
System magnification Ns of an afocal System consisting of a thin spectacle lens and a refraction deficit   Ns  =      1          1      -                        e          *                ⁢                  S          ′                    wherein:    S′ is the vertex power of the spectacle lens    e′ is the distance from the image side vertex of the spectacle lens to the principal plane of the eye.
Shape factor of an afocal System consisting of an “imaginary” afocal spectacle lens having the same front surface power and thickness   Ne  =      1          1      -                        d          n                ⁢                  D          1                    wherein:    D1 is the surface power of the front surface    d is the thickness of the spectacle lens    n is the refractive index of the spectacle lens.For a detailed explanation of the assumptions made and the parameters used, attention is drawn to FIG. 1 in which the model of a centered afocal optical system and the magnifications resulting with this system are graphically illustrated.